Diamond is one of the best characterised allotropes of carbon. It possesses a unique combination of materials properties including the highest known hardness, excellent thermal conductivity, high chemical inertness, good biocompatibility, and a wide optical transmission range. Owing to its extreme hardness, diamond is widely applied in tools as cutting and wear-resistant material. Diamond is also used as an anti-erosion agent in the oil and other industries, as a polishing material in the optics and electronics sector, and has been proposed as a lubricant in vacuum tribology.
Other applications of diamond include its use as a transmission window for lasers, sensing and imaging and heat-spreaders for optoelectronic and semiconductor devices, in electrochemical devices such as electrical double layer capacitors, in micro-electromechanical systems (MEMS), as a medical implant material, as a carrier component in drug delivery systems, and in the nuclear field.
Diamond can also be applied to improve the properties of advanced composites, due to its high hardness and thermal conductivity and/or its low thermal expansion coefficient. For example, the incorporation of nanodiamond powder into organic polymers such as polyvinyl alcohol, polylactide and epoxy leads to improved mechanical properties and thermal conductivity in the composites. Moreover, composites of diamond/Al, diamond/SiC/Al, diamond/Cu, diamond/carbon nanotube and diamond/pyrocarbon are useful in applications such as field emission devices, electronic packaging and heat sinks.
The transformation of graphite into diamond is of great interest to academia and industry and has been the subject of numerous studies for many decades. The phase diagram of carbon shows that diamond is the thermodynamically stable allotrope of carbon at pressures in excess of several GPa over a wide temperature range. However, diamond may also exist as a metastable phase at ambient pressure.
It is possible to convert graphite directly into diamond, but this requires extreme pressures and temperatures to overcome the large activation energy that is necessary for the breaking of the sp2-bonds in the graphite structure and the formation of new sp3-bonds in the diamond structure. The direct transformation of sp2-graphite to sp3-diamond is known to take place at high temperatures and pressures of about 3000° C. and 12 GPa, respectively.
In the 1950s, it was discovered that molten transition metals such as Fe, Co, Ni and their alloys are able to dissolve carbon and then precipitate diamond under the conditions of high pressure and high temperature (HPHT) in its thermodynamically stable region. Typical pressures required are 5 to 6 GPa at temperatures of at least 1300° C. In this process, the metallic medium acts as a solvent-catalyst that reduces the activation energy, and thereby the pressure-temperature conditions, for the graphite-to-diamond transition.
Diamond may be produced at low pressures by means of a chemical vapour deposition (CVD) process. CVD processes deposit diamonds on a substrate using a heated mixture of carbon-containing gas and hydrogen. Gem quality synthetic diamonds have been produced by using diamond seed crystals and a CVD process.
In the 1990s, it was demonstrated that the molten carbonates of Li, Na, K, Cs, Mg, Ca and Sr are also able to act as a solvent-catalyst for diamond formation from graphite at typical HPHT conditions of 5 to 8 GPa and 1600 to 2150° C. Subsequently, several other inorganic melts, including alkali metal halides such as LiCl and mixed systems comprising more than one component, have been used successfully under similar experimental conditions. In all cases, applying HPHT conditions has been considered as a critical prerequisite for the successful transformation.
Theoretical analyses have shown that the sp3-diamond nucleation from sp2-carbon may be more preferable inside a carbon nanotube (CNT) or carbon nanoparticle. This is due to the effect of surface tension induced by the nano-meter sized curvature of such carbon nanomaterials, in comparison with the direct nucleation of diamond from graphite. Many attempts, therefore, have been made to transform chemical vapour deposition (CVD) synthesised CNTs into diamond, mostly using relatively high temperature and/or high pressure techniques such as laser irradiation, shock wave processing, spark plasma sintering, and radio-frequency hydrogen plasma processing. Such methods of transforming CNTs into diamond require complicated and expensive equipment, such as high energy electron/particle beams, spark plasma sintering or a HPHT facility. This reduces the significance of advantages associated with the utilization of CNTs for the synthesis of diamond.